1. Field of the Invention
The present invention relates to a rotary valve and, more particularly, to a rotary valve with a hermetic rotary seal assembly for preventing process fluid which passes through the valve from escaping to the environment. The invention also relates to a hermetic seal assembly for sealingly controlling rotary motion of a member mounted within a valve body or other enclosure.
2. Description of the Background
The valve industry has long recognized that dynamic seals, i.e., seals between sliding and/or rotating components, do not provide the high reliability achieved by static seals. Ball valves, plug valves, butterfly valves, and other rotary valves conventionally each employ a rotary stem extending through the valve body and connected to a valve member within the body to control fluid flow through the valve. Traditionally, a dynamic seal, such as a packing, was provided for sealing between the valve body and the rotary stem. This dynamic seal does not satisfy increasingly stringent government regulations designed to eliminate leaks of process fluids from the valves, and accordingly increased efforts are being expended to provide a valve which does not utilize dynamic seals to eliminate leakage from the valve.
A bellows has long been employed as a viable alternative to avoid dynamic seals. Various valve manufacturers have thus promoted valves which replaced the dynamic stem seal with a bellows to hermetically seal a chamber and thus prevent the escape of process fluids from the valve. Early disclosures of a bellows within a valve enclosing a valve stem or shaft are contained in U.S. Pat. Nos. 785,149; 1,301,948; 1,644,825; 2,659,569; and 3,787,023. Bellows have been used in valves either to seal a shaft with an upper end of a housing (externally pressurized bellows), or to seal a shaft with the valve body (internally pressurized bellows).
In recent years, bellows surrounding a valve stem have been most commonly used in rotary valves to retain pressurized fluid within the valve body and the interior of the bellows. Since the flow path through the valve and the interior of the bellows are in fluid communication, the internally pressurized bellows design avoids problems associated with sealing the chamber external of the bellows from the environment. In order to obtain the necessary rotation of the valve member without rotating the bellows, the upper end of the stem within the bellows is bent or offset with respect to the lower end, so that the upper end may be rotated in an orbital path to rotate the lower end of the stem and thus the valve member. Valves of the type described above are more fully disclosed in U.S. Pat. Nos. 3,782,685; 3,811,651; 4,452,426; and 4,516,574.
One of the problems encountered with a rotary actuator having an internally pressurized bellows surrounding its bent valve stem is referred to as "squirm" of the bellows. A bent, internally pressurized bellows is unstable and thus tends to buckle, especially when the valve and thus the bellows are subjected to high pressure. A second problem concerns a trade-off between the desired high sealing reliability for a bellows, and the expense and size of a large valve actuator. The cycle life of a bellows is directly related to the radius of curvature for the bellows, i.e., its cycle life decreases as its radius of curvature decreases. The length of the bellows is preferably short to minimize manufacturing costs and reduce the overall size of the valve. In some valve designs, the bellows bends a full 90.degree. over a relatively short distance, which adversely affects the life of the bellows and thus the long-term reliability of the valve. Most valves with 90.degree. bent bellows are small, hand-operated valves generally subject to infrequent operation and/or relatively low process line pressure.
Other valves have been designed with bellows which are subject to only a slight bend, so that the moment arm (the radial distance from the bend in the shaft to the valve stem axis) acting on the valve member is relatively short. In this case, a large and more powerful operator typically is used to rotate the upper end of the stem through its 90.degree. turn and thus open or close the valve. Practical size and cost limitations on the operators limit the torque rating of these valves, and valve with bellows having a slight bend and controlled by powered operators are typically installed on process lines having a diameter of 6 inches or less.
Various techniques have been employed, such as those discussed in U.S. Pat. Nos. 4,462,422; 4,468,002; 4,540,019; and 4,546,662, to minimize the squirm of the bellows and increase the bellows life. While these techniques may reduce bellows squirm in a rotary valve, they do not overcome this problem. Moreover, the prior art does not satisfy the need for a large and expensive operator to open or close a sizable valve if the valve is of the type utilizing a bellows to hermetically seal the interior of the valve from the environment.
The disadvantages of the prior art are overcome by the present invention, and an improved bellows sealed rotary valve and a seal assembly for use with such a valve are hereinafter disclosed.